The invention relates to a method for producing glass-coated electronic components and the use of said method for the passivation of electronic components.
Electronic components store, consume or transfer electrical net power, inter alia as passive components, such as electrical resistances, capacitors or also as coils. On the other hand, active electronic components are components which can add electrical net power to a signal. Active components are in particular semiconductors, such as diodes.
Diodes are semiconductor components in the field of electronics, characterised by their asymmetric characteristic current-voltage-curve which strongly depends on the current direction. Diodes are mainly used for rectification of alternating currents.
Common diodes, in particular diodes with little performance, consist of a silicon chip which is pre-passivated at the periphery and which consists of a contact between an n- and a p-semiconductor, connections of copper cover wires or, respectively, copper cover head pins and a glass tube which encapsulates the diode and the connection points. The contact between the chip and the connections is effected by pressure which is maintained by the glass tube.
The term “passivation of electrical components” means, inter alia, the application of a mechanically stable layer onto the final components or their casings.
In this case, the passivation protects the component from mechanical damaging and other detrimental influences caused by impurities, in particular for the further processing. Mostly, passivation of the electric components is effected by applying glass through dripping or vapour deposition. Therefore, passivation constitutes a mechanical protection of electric components, such as diodes and transistors, and, in addition, helps to stabilise the electrical properties. For example, applying the passivation layer is often the final coating step during the production of a semiconductor. In addition, for opto-electric components often a low-reflection layer is applied.
A passivation using glass is generally used for enhancing the quality and the reliability, of inter alia many kinds of Si-semiconductor components and of bipolar ICs up to power rectifiers.
Fused glass layers provide secure protection of the semiconductor surfaces from mechanical and chemical attack both during production and use, as already mentioned above. Due to their barrier and partial getter effect, they can also positively influence the electrical function of the components (blocking voltage, blocking currents).
The thermal expansion of silicon is about 3.3×10−6/K, which is very low. Glasses having a similarly low thermal expansion have high viscosities and thus melting temperatures which are so high that they cannot be considered for use as passivation glasses. Thus, for passivation only special glasses with special properties can be used. The glasses must inter alia have a very good expansion adjustment, good electrical insulation and a dielectrical break-through resistance.
In most production technologies, the glass passivation is followed by glass chemical process steps, such as etching of contact windows and electrodeposition of contacts which can contribute to an attack on the glass. There are high differences in the chemical resistance of passivation glasses that are considered individually when selecting the type of glass.
In the prior art, a selection of standard passivation glasses is available which is used in practice.
U.S. Pat. No. 3,113,878 describes sealing the housing of electric components, the emphasis here being placed on devitrifying the sealing mass. It is true that in the sealing mass, a crystalline phase is present. Passivation of an electronic component with such a mass is not provided for. The crystalline phase can affect functioning of the component.
The passivation by means of glass is also known from EP 091 909 B1, whereby the glass composition as disclosed in particular does not contain ceroxid. Moreover, the composition contains alkali metal oxides having negative effects as these glasses act as ion conductor, in particular with increasing temperatures.
Moreover, the zinc borosilicate glasses described here tend to devitrify (crystallize).
EP 025 187 discloses a composition for ceramic glass frit having a low melting point and being lead-free and little water-soluble. The further field of application of the ceramic composition is in the area of decoration varnishing and in the use as a coating mass for glass and metal.
Chemically and electrically stable glasses from the system PbO—ZnO—SiO2—Al2O3—B2O3 are known from the prior art [M. Shimbo, K. Furukawa, J. Ceram. Soc. Jpn. Inter. Ed. 96, 1988, pages 201-205] and are in the field of electronics amongst others used for the passivation of diodes. In this case lead oxide as a component generates a particularly high electric insulation in the glasses while, however, being environmentally unfriendly.
Zinc borosilicate glasses react most delicately of all glasses to all strong chemicals, such as acids and bases. Therefore, they are only used for vapour deposited contacts, an exception being their use in sinter glass diode technology, wherein during the galvanic tinning of the connection wires, also a noticeable removal of glass takes place. Zero-current nickelization is a special exposure for the passivation glasses. Only lead borosilicate glasses having melting temperatures of ≦700° C. can substantially withstand this procedure.
It is a disadvantage of the known passivization glasses that the majority of them contain a high percentage of lead oxide (PbO).
As lead oxide is—as mentioned before—a component that is detrimental to the environment and as there is legislation underway aiming to prohibit use of this component in electric and electronic apparatuses, there is a need for PbO-free glasses that are amongst others suitable for passivation of electronic components such as semiconductor components, in particular for the use with lead-free diodes.
By simply substituting lead oxide for one or more other ingredients which are sufficiently available, an economical reproduction of the desired technical glass properties which are influenced by PbO cannot be achieved. Methods for passivating electronic components having lead-free passivation glasses that comply with all further requirements to a passivating layer, and which do in particular not tend to devitrify, are not known from the prior art.